Recently there has been a surge in the research and development of Group III-Nitride semiconductor-based, deep ultraviolet (UV) light emitting diodes (LEDs). This surge reflects appreciation of the value of these LEDs in water purification, air purification, epoxy curing, bio-detection and bio-medical applications, and other applications.
There is also interest in better semiconductor devices generally and sensors capable of detecting photons in the 250 A-500 A wavelength range.
Group III nitride compound semiconductors such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and combinations of these such as aluminum gallium nitride (AGN or AlGaN) (hereinafter referred to as a “Group III-nitrides”) have been gaining attention as materials for semiconductor devices that emit light from the green portion of the visible region of the electromagnetic spectrum to the deep ultraviolet region. The term light-emitting device or LED will be used herein to refer to both light-emitting diodes and laser diodes that emit green, blue and ultraviolet radiation, unless otherwise specified. The ultraviolet portion of the electromagnetic spectrum is often subdivided by wavelength into UVA (315-380 nm), UVB (280-315 nm) and UVC (<280 nm).
LEDs are difficult to manufacture for a number of reasons. For example, defects arise from lattice and thermal mismatch between Group III Nitride-based semiconductor layers and substrates such as sapphire, silicon carbide, or silicon on which the LEDs are constructed. In addition, impurities and tilt boundaries result in the formation of crystalline defects. These defects have been shown to reduce the efficiency and effective lifetimes of these LEDs. Furthermore, these same defects have been observed for Group III-Nitride films grown hetero-epitaxially on the above-mentioned substrates, often with typical dislocation densities ranging from 108/cm2 to 1016/cm2 for films grown via metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE) and several other less common growth techniques. Reducing the dislocation density has accordingly become an important goal for improving the quality of LEDs.
One way to reduce the dislocation density is based on the use of epitaxial lateral overgrowth (ELOG), which is a well-known technique. With this method, the dislocation density can be reduced by three or four orders of magnitude, to about 105/cm2 to 106/cm2. This method, however, has been shown to be ineffective for the growth of aluminum-containing Group III Nitride-based semiconductors because of the tendency for the aluminum to stick to masked material and disrupt lateral overgrowth.
Several variations of ELOG have also been demonstrated including lateral growth (PENDEO) epitaxy, and facet-controlled epitaxial lateral overgrowth (FACELO) growth. However, all of these techniques suffer from the same issue plaguing aluminum-containing III-Nitride materials. Additionally, a technique called cantilever epitaxy involves growth from pillars that are defined through etching, as opposed, for example, to masking.
Currently, several research groups are actively developing low-defect density AlN substrates to improve the power-lifetime performance of the deep UV LEDs. There are reports of a new air-bridge-assisted, high-temperature (1500° C.) ELOG approach to deposit 12 μm thick, high-quality AlN layers over SiC substrates as templates for deep ultraviolet LEDs.
Yet another approach to decreasing defect density is a process referred to as pulsed lateral overgrowth (PLOG) wherein pre-formed layers are etched to define islands. By controlling the flow rate of precursor material, a layer coalesces over the islands. Pulsed lateral overgrowth of AlxGa1-xN has previously been demonstrated as an approach for depositing 15-20 μm thick AlxGa1-xN over basal plane sapphire substrates. Instead of the high temperature approach, a pulsed growth mode at 1150° C. was used to enhance Al-precursor mobility over the growth surface. These PLOG AlxGa1-xN layers show a significantly reduced number of threading dislocations (˜107/cm2) in the lateral overgrowth regions, which enables demonstration of optically-pumped lasing at 214 nm. In previous reports, the PLOG AlxGa1-xN was grown either from shallow (−0.3 μm) trenched sapphire or from thin AlN etched templates (˜0.3 μm).
Several other approaches to dislocation reduction have been reported that do not involve selective area growth, such as insertion of an interlayer between the substrate and the semiconductor layer to relieve strain, filtering dislocations by bending them into each other by controlling surface facet formation or by inserting a Group III-Nitride super-lattice layer as described in Applied Physics Letters, Jul. 22, 2002; Volume 81, Issue 4, pp. 604-606, between the buffer layer and the active layer.
Milliwatt-power, deep UV LEDs on sapphire substrates with AlGaN multiple-quantum-well (MQW) active regions have been previously reported for the UVA, UVB and the UVC regions. The LED designs are characterized by an AlN buffer layer deposited using pulsed atomic layer epitaxy (PALE), an AlN/AlxGa1-xN, super-lattice layer between the AlN buffer and an AlGaN n-contact layer for controlling the thin-film stresses and mitigating epilayer cracking; and a p-GaN/p-AlGaN hetero-junction contact layer for improved hole injection.
A majority of the current solutions for defect mitigation involve a superlattice. A superlattice is ultimately either a sacrificial layer or it is left as part of the finished LED but without function. A sacrificial superlattice represents material which must be manufactured and scrapped thereby increasing manufacturing and material cost. The superlattice is also detrimental to device performance because it is insulating and therefore contributes to heat build-up. The thickness deviation of AlN and AlGaN leads to eventual cracking of the superlattice layer due to strain and lattice mismatch. Controlling the thickness of individual layers, the quality of the epilayers and the composition of AlGaN in a superlattice layer is thus a major issue in growing high-quality, defect-free, thick UV LEDs with superlattices.
From the foregoing review, it is clear there has been considerable research expended to find a methods for mitigating defect propagation and strain management.
In spite of advances in material quality and improved manufacturing methods, LEDs and other semiconductor devices that operate in the visible green to ultraviolet region of the electromagnetic spectrum still suffer from difficulties in both materials and manufacturing techniques.